For methods for applying a coating material such as solder resist, etc., to flat-plate type objects to be coated, such as printed circuit boards, etc., there is a method of coating by means of screen printing, a method of coating by means of subjecting an air or airless coater to reciprocation while objects to be coated are being carried by a conveyor, and the like.
These conventional coating methods, however, have the disadvantage that transfer efficiency of the coating material is insufficient, although a relatively uniform distribution of film thickness can be obtained. Furthermore, another disadvantage is that, in the case of coating with an aqueous solder resist coating material, air is caught up to form bubbles, which are contained in the coating.
Moreover, apparatuses such as an air spray electrostatic coating apparatus, an airless spray electrostatic coating apparatus and the like have been proposed, but the above disadvantages have not thoroughly been obviated.
Next, also known is a method of coating by means of subjecting a bell-shaped electrostatic coater used as a rotary spray-type electrostatic coater to reciprocation while the objects to be coated are being carried by a conveyor. In this method, however, it is difficult to obtain a relatively uniform distribution of film thickness because spray patterns are hollow and of large sizes. A proposal has been made to alter those spray patterns by means of shaping air, etc., but this alteration by shaping air, etc., would cause problems such as a drop in transfer efficiency, etc. Furthermore, in the case of coating on both sides of an object to be coated, in a consecutive manner by means of using a bell-shaped electrostatic coater, two such coaters are required; accordingly, an increase in initial cost would become another problem.
Moreover, a method of coating by means of using a disc-shaped electrostatic coater has the disadvantage that, although transfer efficiency is high, some difference in size between flat-plate type objects to be coated would cause film thickness to be non-uniform because those objects to be coated are carried in a circle with the coater as its center. Besides, in the case of double-size coating which is performed while objects to be coated are rotating, problems such as a rise in film thickness on edge portions of the coated objects, etc., would arise.